This invention generally relates to endodontic files designed to be used in a rotary hand piece and whose working portion is made of a shape-memory material such as nickel-titanium alloy. More particularly, this invention relates to a pre-curved rotary endodontic file.
Endodontic files intended for use as manual hand files and made out of stainless steel are almost always pre-curved. However, this type of file could not be used in a hand piece because the file would either cut into the root canal wall or break. When rotated in a direction opposite the cutting direction, the file would tend to deform and straighten.
Endodontic files intended for use in a rotary hand piece and made out of a shape memory material such as nickel-titanium alloy do not make use of a pre-curvature. This type of file is designed to be “true” or straight along its entire active or working length relative to the longitudinal axis of the file. The shape-memory characteristic of the file allows it to combine high strength with high flexibility, allowing the file to traverse the curves of a tooth root canal. However, the file continually wants to straighten itself as it traverses those curves and therefore always urges the working portion of the file against the outside curvature of the root canal wall at the expense of the inside curvature. Torsion forces experienced by the file, as well as friction and heat generated by the file, increase as the amount of surface area of the working length of the file contacts the root canal wall. Additionally, because the curvature of the root canal tends to be irregular, the file can encounter a ledge and bind up or cut into the wall, deviate from the curve, and begin to make its own path. Short radius curvatures tend to more problematic with respect to the above than are longer radius curvatures.
Shape-memory eliminated the breakage problems experienced by stainless steel files, and stainless steel files, because they were not flexible, had to be pre-curved. (However, the pre-curved stainless steel files tended to irreversibly straightened when becoming compressed as they traversed straight portions of the root canal.) To intentionally form a curve in a shape-memory endodontic file runs counter to the very reason that a shape-memory material is preferred for use in the first place. However, U.S. Pat. No. 5,842,861 discloses a set of files made from shape-memory material and having a pre-curvature or “hook” in the last one-third of the file. The hook at the end “makes it easier to direct the file down into the apical region of the root canal, particularly where the root is rather tortuously curved or twisted.” Each pre-curved file in the set of files can have a different degree of curvature so that a desired shape of the apical region of the root canal can be achieved.